CO2-sensing respiratory chemoreceptors are found in diverse locations in air-breathing vertebrates, including multiple sites within the brainstem, carotid and aortic bodies, and lungs, affirming their functional importance in physiological pH/PCO2 homeostasis. We have a long-standing interest in CO2 signal transduction by an unusually responsive type of CO2 sensor -the avian intrapulmonary chemoreceptor (IPC). Past IPC research indicates they (like most respiratory chemoreceptors) probably sense H+ rather than CO2 directly. In IPC, CO2-induced acidosis inhibits action potential discharge rate, making IPC an excellent model for many mammalian central and airway chemoreceptors that share this characteristic, and a good contrast to many other mammalian respiratory chemoreceptors that are excited by CO2. The first aim of this proposal will test the intracellular pH sensing hypothesis using weak and strong acids and bases that vary in their degree of ionization and membrane permeability. Weak acids and bases should have better intracellular access and greater effects on IPC discharge and CO2 sensitivity than strong acids and bases. The second aim investigates TREK-like tandem pore domain leak channels that we recently discovered in IPC. Because TREK channels are opened by intracellular acidosis, they could be the long-sought molecular target for inhibition of IPC by intracellular acidosis/CO2. This will be tested using TREK agonists or antagonists including riluzole, polyunsaturated fatty acids, arachidonic acid, cyclopropane, nitrous oxide, xenon, and osmotic challenges. The third aim will test whether peak IPC discharge rate and the magnitude of IPC spike frequency adaptation (i.e. attributes of neural coding) scale with body size in proportion to M-1/4. Such scaling would match information delivery by IPC afferent discharge to the breathing frequency of the animals, which also scales to M-1/4. To test this, dynamic IPC discharge responses to standardized CO2 step stimuli will be quantified in very small animals (~12 g body mass) and in the very large animals (~100,000 g body mass). This research will enhance understanding of respiratory chemoreceptors in general and may reveal drugs useful for treating CO2 insensitivity during respiratory failure. This research investigates fundamental neural processes that detect CO2 levels in the body and send a neural signal to the brain that ultimately controls breathing. This is important for human health, because understanding the basis of CO2 chemotransduction may help develop more effective treatments and drugs for the loss of CO2 chemosensitivity that often complicates patient survival in serious cardiopulmonary disease. [unreadable] [unreadable] [unreadable]